Vacuum type potting fixture for coating articles



p 4, 1962 J. w. HANZEL 3,051,991

VACUUM TYPE POTTING FIXTURE FOR COATlNG ARTICLES Filed March 26; 1959 INVENTOR. JOSEPH W. HANZEL ATTORNEYS.

United States Patent Office Patented Sept. 4, 1962 3,051,991 VACUUM TYPE PUTTING FDKTURE FUR COATING ARTICLES Joseph W. Hanzel, China Lake, Calif., assignor to the United States of America as represented by the Secretary of the Navy Filed Mar. 26, 1959, Ser. No. 8tl2,271

4 Claims. (Cl. 18-36) (Granted under Title 35, US. Code (1952), see. 266) The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to the art of coating articles; more specifically it rel-ates to apparatus and a method for coating propellant grains.

One convenient method of coating is to place the article to be coated in a mold, pour in a liquid coating material and cure said material so that it becomes a solid, firmly gripping the article. The procedure results, in many cases, in air bubbles being entrapped on the surface of the article which causes void areas to appear in the cured coating.

Propellant grains have been coated with inhibitors by the foregoing technique, but the aforementioned void areas in the coating are undesirable. Elforts to reduce the number of entrapped air bubbles led to the pouring of the coating liquid in fractions, allowing each fraction sufiicient time to settle before adding the next fraction; also creating a vacuum over the mold after pouring in the liquid gave the same effect. Such efforts to remove air bubbles are time consuming however.

It is therefore an object of this invention to provide a method and apparatus for producing bubble-free coatings on objects, especially propellant grains.

Another object is to produce the bubble-free coatings simply and in a relatively short time.

The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawing in which:

FIGS. 1-4 are cross-sectional elevational views of the coating apparatus showing in sequence the operation thereof.

Referring now to the drawing, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIGS. 1-3, a mold 1 consisting of a hollow cylindrical body having a countersink at one end so as to form beveled surface 2, and a closure at the other end, said closure having an aperture 3 therein. Propellant grain 4 is covered partially by the inhibiting solution 5. Centering plug 6 positions the grain 4 inside the mold and also forms a seal to hold back the liquid inhibitor 5. Push-out plug 7, having a centrally positioned aperture extending therethrough and of substantially the same diameter as the centering plug, is positioned directly below said plug in the cylindrical mold. A thin coating of lubricant 8 covers the inner wall of the mold.

In FIG. 4, the inhibitor has been cured and has become solid. Pushrod 9 is insertable into the opening 3 of the mold 1 to bear against plug 7 in expelling the inhibited grain; a portion of the end of pushrod 9 is in sertable in the centrally positioned aperture of plug 7 in order to align pushrod 9 with the longitudinal axis of the mold l.

The apparatus may be made easily and cheaply from many materials including metal, wood, plastic or the like. The only critical part of the apparatus is the centering plug 6 which must form a liquid-tight seal with the inner wall of the mold. Preferably, the centering plug is made of an elastomeric material such as rubber or plastic and is made to have a slightly larger diameter than the diameter of the mold so that the aforementioned seal is formed when the plug is forced into the mold. Alternatively, the centering plug may be made of an inelastic material and the aforesaid seal obtained by use of an O ring of elastic material placed between the plug and the inner wall of the mold. The two aforementioned plugs may be made integral if desired.

The centering plug may be indented to receive the grain or article if desired.

The inhibitors and lubn'cants employed in this invention are conventional and well known in the art.

The foregoing descriptions has been concerned with the preferred embodiment. It is obvious that the mold may have any cross-sectional configuration desired. Another obvious variation is a plurality of such molds joined in a series where a grain is pushed into each mold by a single lever having a corresponding number of small pushrods attached thereto, each push-rod positioned to push on a grain, inhibitor solution poured into each mold from containers positioned above and controlled from a single point, and the cured products removed from the molds by a single lever having a corresponding number of small push-rods attached thereto, each such push-rod positioned to push on a push-out plug.

In operation, the push-out plug 7 is pushed into the mold 1, which has been coated with a lubricant 8, and the centering plug 6 is pushed in on top of the pushout plug so that the top of the centering plug is even with the lower edge of beveled surface 2. The grain 4 is centered on plug 6 and a predetermined amount of liquid inhibitor is poured into the reservoir formed by the beveled surface 2 and the centering plug 6. The grain 5 is then pushed into the mold as far as it will go so that ambient pressure forces the inhibiting solution to fill up the vacuum created by the expulsion of air from the mold through orifice 3. The thickness of the layer of liquid surrounding the grain should be uniform at this point and the level of the liquid should not be above the lower edge of the beveled surface 2; these two conditions are met by control of the thickness of the aforementioned plugs.

Next, the mold and its contents are heated to cure the inhibitor and upon completion of the cure, the coated grain is pushed out of the mold with push-rod 9.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A vacuum-atmospheric pressure diiferential potting fixture for coating a longitudinally extended propellant grain comprising a hollow mold formed with a closure having an orifice therethrough, a bore of constant diameter, the lower end of said mold having the upper end of said mold being exposed to atmospheric pressure and having a frusto-conical surface extending from the outer periphery of the mold to the bore diameter, a push-out plug slideably disposed in said bore, an elastomeric centering plug having a diameter slightly greater than said constant diameter and slideably disposed in said bore above and adjacent to said push-out plug for forming a liquid-tight seal with said bore, said frusto-conical surface and the upper surface of said centering plug defining a reservoir for receiving a given volume of liquid inhibiting material such that the longitudinally extended side and upper surface of said grain is adapted to be covered by said inhibiting material when said grain is pushed to the bottom of said mold, the pressure differential between the vacuum created adjacent the u per surface of said elastomeric centering plug and the atmospheric pressure at the surface of said liquid inhibiting material causing said liquid inhibiting materal to surround said grain and being free of air bubbles at the surface of said grain upon pushing the grain downwards, said orifice adapted to receive a push-out rod for engaging said push-out plug and pushing the coated grain out the upper end of said mold.

2. In a means for coating the sides and top of a propellant grain; in combination, a cup-shaped member having a vertical bore, the lower end of said member formed with a closure having an orifice in communication with the lower end of said bore, the upper end of said member formed with a beveled surface flaring outwardly from said bore; a push-out plug disposed for sliding in said bore; a centering plug disposed atop said push-out plug for sliding in said bore in liquid-tight sealing engagement therewith, said centering plug adapted to support a propellant grain of predetermined length centrally of said bore, said grain having lateral dimensions less than the lateral dimensions of said bore, said bore having a vertical extent exceeding the sum of the vertical dimensions of said plugs and grain by an amount equal to the thickness of the coating for the top of the grain; said beveled surface with said grain forming a reservoir adapted to receive a predetermined amount of coating material sufiicient to coat the sides and top of the grain; and said orifice being adapted to vent said bore and receive a push rod for pushing the coated grain upwardly out of the bore.

3. A method for uniformly coating the sides and top of an object with a liquid material, comprising the steps of (1) providing a hollow mo ld having a countersink at the top thereof with a centering member slidable within the mold in sealed relation thereto and defining with the countersink a reservoir, (2) centering the object to be coated on the member, (3) placing an amount of liquid material in the reservoir sufficient for coating the sides and top of the object with a uniform thickness of the material, (4) moving the member and object, while maintaining a vacuum at the lower end of the object, inwardly into the mold to the point where the top of the object is covered with the material and is below the countersink a distance equal to the thickness of the coating on the sides of the object, and moving the coated object out of the top of the mold.

4. A method for coating the sides and top of a solid propellant grain with a uniform thickness of inhibiting material, comprising the steps of (1) providing a hollow mold having a countersink at the top thereof with a sealing plug slidable within the mold and defining a reservoir with the countersink, (2) centering the grain on the plug, (3) placing an amount of inhibiting material in liquid form in the reservoir suflicient for coating the sides and top of the grain with a uniform thickness of the material, (4) moving the plug and grain, while maintaining a vacuum at the lower end of the grain, inwardly into the mold until the top of the grain is covered with a thickness of material equal to the thickness of the coating on the sides of the grain, (5) heating the coated grain in place, and moving the coated grain out of the top of the mold.

References (Jited in the file of this patent UNITED STATES PATENTS 289,037 Shickle NOV. 27, 1883 879,915 SChrOder Feb. 25, 1908 2,570,284 StOtt et al. Oct. 9, 1951 2,799,049 Wilson July 16, 1957 FOREIGN PATENTS 388,662 Great Britain Mar. 2, 1933 

